Polycrystalline silicon (polysilicon) is widely used as a means of making contact to the various structures of bipolar and MOS devices. For example, polysilicon film is utilized in advanced integrated circuit technology as a diffusion source, as an emitter contact in bipolar devices, and as a buried contact in MOS devices. In these instances, the polycrystalline silicon is usually deposited on a monocrystalline silicon substrate. It is widely known that interfacial oxide at the polysilicon/monocrystalline interface strongly affects the electrical characteristics of semiconductor devices resulting therefrom, because the presence of this interfacial oxide affects the transport of dopants and the electrical transport of carriers across the interface. For example, it is known that the presence of an oxide layer at the interface between polycrystalline silicon and the monocrystalline silicon in the emitter of a polysilicon emitter bipolar transistor will diminish the transistor's base current and hence will increase its current gain, beta. (Current gain of bipolar transistors is defined as the ratio of collector current to base current). Suggested explanations for these observations have included reduced emitter band gap narrowing, tunneling transport of electrons and holes across the oxide layer, and minority carrier transport effects on the heavily doped polysilicon layer.
If high gain polysilicon emitter transistors incorporating an interfacial oxide layer are to be used in commercial integrated circuit processes, it is important that the oxide layer be fully characterized in terms of thickness and uniformity and that it is stable during subsequent high temperature processes.
Although there are advantages to having the interfacial oxide layer, there are disadvantages also, including the fact that it increases the transistor emitter resistance, thereby reducing its collector current and consequently the performance of the transistor. It is therefore important to carefully control the interfacial oxide layer if it is to be used effectively. Prior attempts to realize the type of control necessary for effective manufacturing processes have been unsuccessful. For this reason recent disclosures have suggested removing entirely the interfacial oxygen from the interface by utilizing rapid thermal annealing techniques. Example of such disclosure may be found in Turner, et al., "Interface Control in Double Diffused Polysilicon Bipolary Transistors", IEEE 1990 Bipolar Circuits and Technology Meeting 1.4, pg. 33; Delfino, et al., "Polycrystalline Silicon Emitter Contacts Formed by Rapid Thermal Annealing", J. Electrochem. Soc., Vol. 136, No. 1, January 1989, pg. 215; and Bravman, et al., "Structure and Morphology of Polycrystalline Silicon-Single Crystal Silicon Interfaces", J. Appl. Phys. 57(8), Apr. 15, 1985.
Completely removing the interfacial oxygen interface as suggested, though, drastically reduces the current gain, beta, of the transistor. Furthermore, the prior art has not disclosed or taught a method for consistently producing an interface with a desired, constant physical structure as well as devices with desired electrical parameters.
A method for accurately controlling the interfacial oxide layer between polysilicon/monocrystalline silicon interfaces, is therefore, highly desirable.